1. Field of the Invention
The invention concerns a cone disc continuously-variable transmission with pairs of cone discs mounted on drive and driven shafts, with one of the cone discs being fixed on the shaft and one of each being axially movable on the shaft, and between which a traction train circulates for power transfer. Together with pistons fixed on the shafts, the axially movable cone discs form cylinder-piston-aggregates. The cylinder-piston-aggregates are supplied by a pump via a control valve with a pressure medium for the setting and maintenance of the gear ratio. The control pressure actuating the control valve is determined as needed by a proportional pressure reducing valve (PV) which is influenced by a control and computer unit which in turn receives the necessary gear ratings from sensors and converts these into appropriate electrical signals. The control pressure acts in a pressure chamber against a piston valve supported by spring elements.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Gears of this class with hydraulic adaptation of the cone discs are known in their basic technology, for instance, through German patent No. DE-A-12 64 196. There the ratio setting is made by mechanical control of the position of the piston valve. A further development of such transmissions with hydraulic contact pressure on the cone discs and also with hydraulically controlled gear ratio setting is known for instance from German patent No. DE-A-39 14 792.
With gears of the latter type, it is possible to have a failure of control pressure which is necessary for the adjustment of the piston valve position and which controls the ratio setting of the transmission. This is so because this control is made with an electronic computer and control unit which acts appropriately on the proportional pressure reducing valve (PV). During a power failure, for instance, this control cannot be maintained. The piston valve moves in an uncontrolled manner, and the control pressure is lost.
During such a pressure failure, the piston valve is moved by the associated spring into an end position which corresponds to a command for a sudden change of the set gear ratio into an extreme position. Depending on the hydraulic flow in the pressure lines from the control valve to the set of drive discs or driven discs respectively, in accordance with the actual requirements in the desired application, this may cause a change of the gear ratio either to acceleration or to deceleration.
Such an involuntary change of gear ratio can cause considerable problems. If, for instance, the drive is connected to a major inert mass, a change command to “Slow” would mean a possibly significant acceleration of the drive engine for the drive side. Applied to the motor vehicle drive, this signifies, for example, that during a sudden gear ratio change to “Slow” (comparable to a manual transmission being suddenly shifted into first gear during high speed), the entire vehicle, acting as an inert mass, forces the engine to high or excessive RPM. This can cause not only damage to the engine but also to the entire drive train.
And it should also be mentioned that such an occurrence on a motor vehicle could also have traffic consequences, when due to the sudden braking, the vehicle gets out of control. It could also lead to a wrong reaction on the part of the driver of the vehicle, with corresponding consequences.
A change to “Fast”, on the other hand, can lead to an overload on parts of the drive train, if, for instance, the driving motor is a synchronous machine tied into the power circuit.
One can also think of applications where large inert masses are present on the drive side and on the power take-off side.